She was in court when her father, Abdulhadi al-Khawaja, one of the country's leading rights activists, was given a life sentence, her younger sister, Maryam, said.

"Right after they read out the verdicts my father shouted, 'The struggle will continue,'" said Maryam, 23, who left Bahrain in March and has been protesting from overseas, tweeting as maryamalkhawaja. "He was beaten and forcefully removed from the court. My sister stood up and chanted, 'Allahu akbar' [God is great], and she was forcefully removed from the court and arrested. She was charged with contempt of court but then was made to sign a pledge not to speak in court again and then she was released."

With official pressure building up on family members within Bahrain – Zainab was briefly arrested last week – it is now largely up to her younger sister, an activist with the Bahrain Centre for Human Rights, to sustain the pressure on authorities.

"It's been very difficult. I'm abroad and my whole family are in Bahrain so I'm on my own. I have not been able to speak to my father since he was arrested," Maryam said. " from my family that He's been suffering a lot from the torture he was subjected to. Two days ago his face was still swollen because of fractures."

Dissent is something of a family trade for Maryam, the second-youngest of four sisters. Her uncle, Salah al-Khawaja, Abdulhadi's younger brother, was jailed for five years. Zainab's husband, Wafu Almajed, and Hussein Ahmed, the fiance of the youngest sister, 21-year-old Batool, are in custody but have not been charged.

The girls' mother, meanwhile, was fired from her job at school, where she worked for 10 years, on orders from the interior ministry, Maryam said.

"We've definitely been targeted as an entire family. But this isn't something that my family has been solely subjected to. Many, many families in Bahrain are experiencing this," she said. "As a human rights activist I always remind myself that my father is not the only one in prison, he's not the only one being tortured, and I have a responsibility not only to work for my father's release but for all political prisoners in Bahrain."

The sisters grew up in Denmark, where the family had political asylum before returning to Bahrain in 2001.

"The most difficult part of coming back was having to see my father get beaten up several times," Maryam said. "He was in and out of prison a lot of the time. We lived in this constant fear that there would come a day when he would not come home, because we knew there had been cases where people were forcefully disappeared.

"In 2004 he was taken from our house and for about three days the authorities refused to tell us where he was, and we thought he was dead. After that we found he was in prison. He was kept there for about two-and-a-half months.

"It was definitely different. In Denmark, as you can imagine, it was a lot safer. Living in Bahrain there's this state of constant worry, constant stress about what's going to happen to my father."

Maryam left Bahrain for a rights conference in Geneva and decided it would be safer not to return. She moves regularly and prefers not to say where she is at any time.

"One of the worst things is having to hear news through Twitter. I was waiting this morning for my sister to tell me what had happened with my father and suddenly I get a call from a reporter saying he'd been jailed for life and Zainab was arrested."

New technology makes such remote activism possible, she said, but also brings its own difficulties: "It's now a lot easier for the government to find out where I am, what I'm doing. I get quite a few death threats on Twitter."